1. Field of the Invention
The present invention relates to a scanning interferometer for spectrochemical analysis, and more particularly to a double beam interferometer which uses the refractive scanning method and is suitable for use in an ultrared Fourier-transform interferometer.
2. Description of the Prior Art
In a conventional scanning interferometer for spectrochemical analysis, a reflecting mirror is moved along an optical path to carry out the optical path length scanning, and therefore optical matching is not always maintained.
In order to solve this problem, the refractive scanning method has been proposed, in recent years. The advantages of this method over the conventional method in which a reflecting mirror is moved, are described in detail in, for example, an article by Doyle (Applied Spectroscopy, Vol. 53, No. 5, 1980, pages 599 to 603). Further, in a U.S. Pat. No. 4,265,540 to Doyle, it is described that a retoro-reflector such as a corner mirror or right-angled roof mirror is provided to reduce the adverse effect of the chromatic aberration caused by a wedge-shaped refractor for refractive scanning, and the retoro-reflector produces an additional effect that optical matching becomes unnecessary.
FIG. 1 shows a conventional interferometer disclosed in the above-referred U.S. patent. Referring to FIG. 1, a fixed optical wedge 32 and a movable optical wedge 28 are disposed face to face with each other in such a manner that the apex angle 6 of the optical wedge 32 and the apex angle 7 of the optical wedge 28 are formed on the same side and facing surfaces of the optical wedges 32 and 28 are parallel to each other. Of the facing surfaces, that surface of the fixed optical wedge 32 which faces the movable optical wedge 28, is a semitransparent mirror surface 12. The movable optical wedge 28 can move along a plane containing that surface of the movable optical wedge 28 which faces the fixed optical wedge 32. The movement of the movable optical wedge 28 is controlled by a driving mechanism 30. Further, in a space on the fixed optical wedge side are disposed a light source 10 and a retoro-reflector 16 formed of a corner mirror. While, in a space on the movable optical wedge side are disposed a detector 24 and a retoro-reflector 18 formed of a corner mirror.
In the above-mentioned arrangement, light 14 from the light source 10 is incident on and refracted by the fixed optical wedge 32, and is then divided by the semitransparent mirror surface 12 into two parts, one of which is reflected from the mirror surface 12 and the other passes through the mirror surface 12. The light reflected from the semitransparent mirror surface 12 leaves the fixed optical wedge 32 in a direction perpendicular to the optical wedge 32, and is then incident on and reflected back from the retoro-reflector 16. The reflected light from the reflector 16 passes through the fixed optical wedge 32, the movable optical wedge 28 and a condenser system 65, and is then detected by the detector 24. While, the light having passed through the semitransparent mirror surface 12 passes through the movable optical wedge 28 and leaves the wedge 28 in a direction perpendicular thereto. The light emerging from the optical wedge 28 is incident on and reflected back from the retoro-reflector 18. The reflected light from the reflector 18 passes through the movable optical wedge 28, and is reflected from the semitransparent mirror surface 12. The reflected light from the surface 12 passes through the movable optical wedge 28 and condenser system 65, and is then detected by the detector 24.
In the above-mentioned construction, the movable optical wedge 28 is moved in directions of the arrow to vary an optical path length 22a in the movable optical wedge 28, thereby scanning the difference between an optical path length 20a in the fixed optical wedge 32 and the optical path length 22a.
However, when light having various wavelength components impinges upon the movable optical wedge 28 in the above-mentioned arrangement, various light components pass through the optical wedges 32 and 28, and are then incident on and reflected back from the retoro-reflector 18. Now, let us consider only two light components having different wavelengths .lambda..sub.1 and .lambda..sub.2. As shown in FIG. 2, the two light components are different in angle of refraction from each other in accordance with the wavelength difference at the movable optical wedge 28, and therefore leave the optical wedge 28 as a light beam 2 and a light beam 3, which are reflected back, as light beams 4 and 5, from the retoro-reflector 18. That is, owing the above-mentioned dispersion, the two light beams 4 and 5 deviate from each other in a direction perpendicular to an optical axis, and thus it is not possible to prevent chromatic aberration completely. Incidentally, the fixed optical wedge 32 is omitted in FIG. 2, for brevity's sake.